Got an Answer?
Maybe when they start fighting put some of your music on to cover the noise? Personally, I like the wiggles or the hooley dooleys. Both are just annoying enough to get stuck in the neighbours heads, but not obnoxious enough to seem malicious.
I've also made it a point to teach my kids about adult words and how they're not to say them. Some words are only for adults and if I hear any of my 4 saying them, I'll wash their filthy mouths out with soap and holy water (something my mammy used to tell us growing up).
I know that at 4 years old, he's still a baby in your eyes, but he's old enough to understand that these people are behaving inappropriately. I think you can sit down and have a good talk with him about it.